docs: def split_coords_2d(seq):
      """
      :param seq: a flat list with lons and lats
      :returns: a validated list of pairs (lon, lat)
  
      >>> split_coords_2d([1.1, 2.1, 2.2, 2.3])
      [(1.1, 2.1), (2.2, 2.3)]
      """
      lons, lats = [], []
      for i, el in enumerate(seq):
          if i % 2 == 0:
              lons.append(valid.longitude(el))
          elif i % 2 == 1:
              lats.append(valid.latitude(el))
      return list(zip(lons, lats))